It has been a practice that tobacco materials are expanded in order to save the amount of the tobacco materials used in tobacco articles such as cigarettes, and to moderate the flavor and taste, etc., of the tobacco articles. This expansion is a technique of allowing the dried and shrunk tobacco tissue to be restored to a state close to that of the live tobacco leaf, and constitutes an important technique in the manufacture of tobacco articles.
In principle, the expansion of the tobacco material is effected by allowing an expanding agent to permeate into the tobacco tissue, followed by heating the tobacco material so as to cause the volume of the expanding agent to be expanded, thereby expanding the shrunk tobacco tissue.
As such a method of expanding tobacco material, there is known a method using carbon dioxide as an expanding agent.
For example, Japanese Patent Publication (Kokoku) No. 56-50830 discloses a method wherein a tobacco material is immersed in a liquid carbon dioxide under a pressure of, for example, about 24.6 to 31.6 kg/cm.sup.2 to allow the liquid carbon dioxide to be impregnated into the tobacco material, followed by converting the impregnated liquid carbon dioxide into solid carbon dioxide and subsequently evaporating the solid carbon dioxide under high temperatures so as to expand the tobacco tissue. In this method, the entire tobacco material is immersed in the liquid carbon dioxide, with the result that the flavor components of the tobacco material are extracted into the liquid carbon dioxide, lowering the flavor and taste of the expanded tobacco material. Further, a large amount of the liquid carbon dioxide attached to the tobacco material is converted into the solid carbon dioxide, with the result that the tobacco material is solidified and firmly consolidated. The consolidated tobacco material requires a considerably large force to loosen it before the expansion step under heat, resulting in the generation of fine particles unsuitable for the manufacture of cigarettes, leading to a low yield. To overcome this difficulty, it is recommended to drip the liquid carbon dioxide off the tobacco material after the immersion of the tobacco material in the liquid carbon dioxide until the liquid carbon dioxide ceases to form a continuous liquid stream. In this case, however, an additional time is required for dripping off the liquid carbon dioxide, and a satisfactory result cannot still be obtained.
Japanese Patent Publication No. 56-50952 discloses a method wherein carbon dioxide in the form of gas is impregnated into tobacco material, followed by rapidly heating the resultant tobacco material to effect the expansion (expanding). This expansion method using the gaseous carbon dioxide certainly permits avoiding the above-noted difficulty inherent in the technique of using a liquid carbon dioxide described above. However, since only a small amount of carbon dioxide is retained in the tobacco material, the carbon dioxide tends to be escaped off before the step of expanding under heat, resulting in failure to expand sufficiently the tobacco material.
Further, Japanese Patent Disclosure (Kokai) No. 4-228055 and Japanese Patent Disclosure No. 5-219928 disclose a method of expanding the tobacco material, in which the tobacco material is sufficiently cooled in advance to increase the amount of impregnated carbon dioxide by condensing the carbon dioxide gas. More specifically, in the method disclosed in Japanese Patent Disclosure (Kokai) No. 4-228055, the tobacco material is cooled by contacting and mixing it with a misty cold mixture comprising cold gaseous carbon dioxide, carbon dioxide snows, and the like that is formed by, while transferring the tobacco material, supplied in a horizontal mixing tank, within the tank, introducing a liquid carbon dioxide into the mixing tank to expand the liquid carbon dioxide. The cooled tobacco material is introduced into a vertical pressure tank connected to the mixing tank and is brought into contact within the pressure tank with the gaseous carbon dioxide so as to achieve the desired impregnation. In this method, a special apparatus is required for the preliminary cooling. In addition, the heat exchange (heat transmission) state between the misty cold mixture (mainly snows) and the tobacco material tends to take place locally, giving rise to non-uniform tobacco temperature distribution. On the other hand, in the method disclosed in Japanese Patent Disclosure No. 5-219928, the tobacco material is cooled preliminarily by allowing a carbon dioxide gas to flow through the tobacco material. This preliminary cooling necessitates the carbon dioxide gas to circulate within a pressure vessel, making it necessary to use an additional circulating equipment. Further, since the carbon dioxide gas used for cooling the tobacco material has a small sensible heat (specific heat), the tobacco material must be brought into contact with a large amount of the carbon dioxide gas, in this method, in order to cool the tobacco material to a sufficiently low temperature. Further, in these prior art methods, since the cooling efficiency of the tobacco material is low, a large amount of carbon dioxide is required for cooling the tobacco material. In addition, even if the tobacco material is preliminarily cooled, the tobacco material is warmed by the compression heat generated when the carbon dioxide gas is boostered to the impregnation pressure for the impregnation of the carbon dioxide gas within the pressure vessel. It follows that it is necessary to preliminarily cool the tobacco material to a low temperature lower than necessary, which is not economical.
It is an object of the present invention to provide a method of producing an expanded tobacco material, which makes it possible to permit a tobacco material to be impregnated sufficiently with carbon dioxide in a short time using a minimum amount required of carbon dioxide and which also makes it possible to produce a high quality expanded tobacco material having a high expansion rate by using an apparatus of a simple construction.